Abstract: UF4 is a key intermediate product that can be converted to UF6 for isotope separation or uranium metal used in nuclear fuel elements. The production of UF4 includes wet and dry processes. The wet process is of a long process and high cost, along with a large amount of waste liquid. Therefore, the dry process is usually used to prepare UF4 in the process of uranium chemical production. In earlier time, UF4 was prepared by the method of onestep reaction of CCl2F2 and UO3. However, Freon has been completely banned as a result of its increasingly apparent damage to ozone. Twostep reaction is usually adopted in the present preparation process of UF4, where UO3 is reduced to UO2 by H2 at high temperature and then fluoridated by HF to produce UF4. This routine provides a high fluorination efficiency, while hydrogen and hydrogen fluoride gas is flammable, explosive and highly corrosive, presenting high demands on productive conditions, such as structure of the factory and material of equipment. NH4HF2 has the advantages of economy, low corrosiveness and low environmental pollution, and is regarded as the most promising fluorination agent. The onestep synthesis of UF4 by fluorination of NH4HF2 has a complex reaction process and many intermediate products, while there are few reports on the intermediate process of it. Therefore, in this work, preparation of UF4 by fluorination of NH4HF2 under different reaction temperatures and proportion of fluoride agent were investigated using a selfmade fluorination reactor. The influence of reaction temperature and proportion of NH4HF2 on the efficiency of fluoride was obtained. The reaction products were identified by the methods of chemical analysis and Xray powder diffraction. Results show that the content of UF4 in products increases gradually with the excess ratio of ammonium hydrogen fluoride at T=573, 673 and 823 K, and the fluoride rate increases first and then decreases with the increase of the excess ratio of NH4HF2 at T=723 K and 773 K. When the temperature is at 573 K to 823 K, the fluorination products have five forms: 4NH4F·UF4, NH4F·3UF4, 3NH4F·UO2F2, UO2F2 and UF4. When the temperature T is not lower than 673 K, there is no double salt 3NH4F·UO2F2 formed, when the temperature is not lower than 723 K, the double salt 4NH4F·UF4 and NH4F·3UF4 disappear, and it only exists in the forms of UO2F2 and UF4 in the products. Thermodynamic basic equation was used to calculate the reaction enthalpy and the equilibrium constant, and it shows that the process using NH4HF2 and UO3 to prepare UF4 is an exothermic reaction.